Wikipedia's category system can be thought of as consisting of overlapping trees. Any category may branch into subcategories, and it is possible for a category to be a subcategory of more than one parent. (A is said to be a parent category of B when B is a subcategory of A). Mathematically speaking, this means that the system approximates a directed acyclic graph.

There is one top-level category, Category:Contents. All other categories are found below this. Hence every category apart from this top one must be a subcategory of at least one other category.

There are two main kinds of category:

Topic categories are named after a topic (usually sharing a name with the Wikipedia article on that topic). For example, Category:France contains articles relating to the topic France.

Set categories are named after a class (usually in the plural). For example, Category:Cities in France contains articles whose subjects are cities in France.

Sometimes, for convenience, the two types can be combined, to create a set-and-topic category (such as Category:Voivodeships of Poland, which contains articles about particular voivodeships as well as articles relating to voivodeships in general).

Which pages a given category is supposed to contain is generally clear from the name of the category. Additional explanation may be placed in the text of the category page.

Pages are not placed directly into every possible category, only into the most specific one in any branch. This means that if a page belongs to a subcategory of C (or a subcategory of a subcategory of C, and so on) then it is not normally placed directly into C. For exceptions to this rule, see Eponymous categories and Non-diffusing subcategories below.

Every Wikipedia article should belong to at least one category. Normally a new article will fit into existing categories – compare articles on similar topics to find what those categories are. If you think a new category needs to be created, see What categories should be created below. If you don't know where to put an article, add the {{uncategorized}} template to it – other editors (such as those monitoring Wikipedia:WikiProject Categories/uncategorized) will find good categories for it.

Each article should be placed in all of the most specific categories to which it logically belongs.

It should be clear from verifiable information in the article why it was placed in each of its categories. Use the {{Category unsourced}} template if you find an article in a category that is not shown by sources to be appropriate, or the {{Category relevant?}} template if the article gives no clear indication for inclusion in a category.

Categorize articles by characteristics of the topic, not characteristics of the article. A biographical article about a specific person, for example, does not belong in Category:Biography.

Articles on fictional subjects should not be categorized in a manner that confuses them with real subjects.

An article should never be left with a non-existent (redlinked) category on it. Either the category should be created, or else the link should be removed or changed to a category that does exist.

By convention, category declarations are placed at the end of the wikitext, but before any stub templates (which themselves transclude categories) and interlanguage links.

The order in which categories are placed on a page is not governed by any single rule (for example, it does not need to be alphabetical, although partially alphabetical ordering can sometimes be helpful). Normally the most essential, significant categories appear first. If an article has an eponymous category (see below), then that category should be listed first of all. For example, Category:George Orwell is listed before other categories on the George Orwell page.

If the desired display text in a particular category is different from the title of the article, it may be appropriate to categorize a redirect, either instead of or along with the article itself. For example, if John Smith was notable as both a musician and a writer, but used the stage name Johnny Rocket in his musical career, then the musician category declarations can be placed on the "Johnny Rocket" redirect page as well as (or instead of) on "John Smith". (Redirects appear in italics in category listings.) See also Wikipedia:Categorizing redirects.

Categories should be useful for readers to find and navigate sets of related articles. They should be the categories under which readers would most likely look if they were not sure of where to find an article on a given subject. They should be based on essential, "defining" features of article subjects, such as nationality or notable profession (in the case of people), type of location or region (in the case of places), etc. Do not create categories based on incidental or subjective features. Examples of types of categories which should not be created can be found at Wikipedia:Overcategorization.

Remember that categories are not the only means of enabling users to browse sets of related articles. Other tools which may be used instead of or alongside categories in particular instances include lists and navigation boxes. For a comparison of these techniques, see Categories, lists and navigation templates.

Categorizations appear on pages without annotations or referencing to justify or explain their addition; editors should be conscious of the need to maintain a neutral point of view when creating categories or adding them to articles. Categorizations should generally be uncontroversial; if the category's topic is likely to spark controversy, then a list article (which can be annotated and referenced) is probably more appropriate.

Before creating a new category, check whether a similar category already exists under a different name (for example, by looking on the likely member pages or in likely parent categories).

Many subcategories have two or more parent categories. For example, Category:British writers should be in both Category:Writers by nationality and Category:British people. When making one category a subcategory of another, ensure that the members of the first really can be expected (with possibly a few exceptions) to belong to the second also. Category chains formed by parent-child relationships should never form closed loops. If two categories are closely related but are not in a subset relation, then links between them can be included in the text of the category pages.

A category may be diffused using several coexisting schemes; for example, Category:Albums is broken down by artist, by date, by genre etc. Metacategories may be created as ways of organizing schemes of subcategories. For example, the subcategories called "Artistname albums" are not placed directly into Category:Albums, but into the metacategory Category:Albums by artist, which itself appears in Category:Albums.

It is possible for a category to be only partially diffused – some members are placed in subcategories, while others remain in the main category.

Information about how a category is diffused may be given on the category page. Categories which are intended to be fully broken down into subcategories can be marked with the {{catdiffuse}} template, which indicates that any pages which editors might add to the main category should be moved to the appropriate subcategories when sufficient information is available. (If the proper subcategory for an article does not exist yet, either create the subcategory or leave the article in the parent category for the time being.)

To suggest that a category is so large that it ought to be diffused into subcategories, you can add the {{verylarge}} template to the category page.

Not all subcategories serve the "diffusion" function described above; some are simply subsets which have some special characteristic of interest, such as Best Actor Academy Award winners as a subcategory of Film actors, Toll bridges in New York City as a subcategory of Bridges in New York City, and Musical films as a subcategory of Musicals. These are called non-diffusing subcategories. They sometimes provide an exception to the general rule that pages are not placed in both a category and its subcategory: there is no need to take pages out of the parent category purely because of their membership of a non-diffusing subcategory. (Of course, if the pages also belong to other subcategories that do cause diffusion, then they will not appear in the parent category directly.)

Often an article and a topic category will share the same name, as in George W. Bush and Category:George W. Bush, or occasionally similar names referring to the same thing, as with Mekong and Category:Mekong River. Such a category is called an eponymous category. Usually the article itself will be a member of the category (and should be sorted to appear at the start of the listing, as described below under Sort keys). A clear link to the main topic article from an eponymous category page can be created using the template {{cat main}}.

The question arises as to whether eponymous categories should be placed in (made subcategories of) the categories which their corresponding articles belong to. Logically they usually should not (for example, France belongs to Category:European countries, but Category:France does not constitute a subset of European countries). However, by convention, many categories do contain their articles' eponymous categories as subcategories, even though they are not "true" subcategories. In any case, an article should not be excluded from any set category on the grounds that its eponymous category is made a "subcategory" of that category.

Sort keys are sometimes needed to produce a correct ordering of member pages and subcategories on the category page. For the mechanics, see Sort order on the help page.

Because the software uses Unicode rather than true alphabetical ordering (see details), it is important that sort keys be capitalized consistently. By convention, the first letter of each word in a sort key is capitalized, and other letters are lower case. For example, use "Dubois" in sort keys rather than "DuBois".

Categories of people are usually sorted by last name rather than first name, so "surname, forename" sort keys are used (as in "Washington, George"). For more information, see Ordering names in a category in the people categorization guideline.

Entries containing modified letters should be sorted as if the letters were unmodified (for example, Łódź has the sort key "Lodz").

Other sort key considerations:

Leading articles—a, an, and the—are one of the most common reasons for using sortkeys. Please also apply these sort keys to deliberate misspellings of these words—e.g. "da" or "tha" for "the."

Entries containing numbers sometimes need special sort keys to ensure numerical rather than alphabetical ordering (for example, 19 and 103 come before 2 in alphabetical order, and IX comes before V). So Haydn's 13th symphony might have the sort key "Symphony 013", the zero ensuring that it is listed before symphonies 100–108; Pope John IX might have a sort key "John 09". It is important to stick to the same system for all similar entries in a given category.

Systematic sort keys are also used in other categories where the logical sort order is not alphabetical (for example, individual month articles in year categories such as Category:2004 use sort keys like "*2004-04" for April). Again, such systems must be used consistently within a category.

Use a space as the sort key for an article matching an eponymous category, or for a key article for the category. For example, the article Barack Obama includes the category sort key [[Category:Barack Obama| ]]. This places the article at the start of the listing for that category. (Note: if the key article should not be a member, simply list it in the category text itself.) Typically, these eponymous categories are listed first even if they do not appear first in alphabetical order.

Use other sort keys beginning with a space (or an asterisk or a plus sign) for any "List of ..." and other pages that should appear after the key article and before the main alphabetical listings. The same technique is sometimes used to bring particular subcategories to the start of the list.

To place entries after the main alphabetical list, use sort keys beginning with tilde ("~"). Other characters used for this purpose are "µ", commonly used to place stub categories at the end of subcategory lists; "β" for book subcategories; and "τ" for templates.

If a page is to be given the same sort key in all or several of its categories, the {{DEFAULTSORT}}magic word can be used. Conventionally, this is placed just before the list of category declarations. Default sort keys are sometimes defined even where they do not seem necessary—when they are the same as the page name, for example—in order to prevent other editors or automated tools from trying to infer a different default.

Rather than leave the text of a category page empty (containing only parent category declarations), it is helpful – to both readers and editors – to include a description of the category, indicating what pages it contains, how they should be subcategorized, and so on. The description can also contain links to other pages, in particular to other related categories which do not appear directly as subcategories or parent categories, and to "sister categories" on other projects, such as Commons.

A maximum of 200 category entries are displayed per screen. To make navigating large categories easier, a table of contents can be used on the category page. The following templates are some of the ways of doing this:

Subcategories are split alphabetically along with the articles, which means that the initial screen of a split category may not include all its subcategories. To make all subcategories display on each screen, add a category tree to the text of the category page, as described at the help page under Displaying category trees and page counts.

Interlanguage links work on category pages just as they do for articles, and can be used to link to corresponding categories on other language Wikipedias.

content categories, intended as part of the encyclopedia, to help readers find articles, based on features of the subjects of those articles;

project categories, intended for use by editors or by automated tools, based on features of the current state of articles, or used to categorize non-article pages.

Project categories include stub categories (generally produced by stub templates), maintenance categories (often produced by tag templates such as {{cleanup}} and {{fact}}, and used for maintenance projects), WikiProject and assessment categories, and categories of pages in non-article namespaces.

Article pages should be kept out of project categories if possible. For example, the templates that generate WikiProject and assessment categories should be placed on talk pages, not on the articles themselves. If it is unavoidable that a project category appears on article pages (usually because it is generated by a maintenance tag that is placed on articles), then in most cases it should be made a hidden category, as described under Hiding categories below.

Category tags can be added to image pages too. Images are not included in the count of articles in the category, but are displayed in a separate section with a thumbnail and the name for each. A category can mix articles and images, or a separate image category can be created. An image category is typically a subcategory of the general category about the same subject, and a subcategory of the wider image category, Wikipedia images. To categorize a new image when uploading, simply add the category tag to the upload summary.

Similarly, user subpages that are draft versions of articles should be kept out of content categories. If you copy an article from mainspace to userspace and it already contains categories, remove them or comment them out. Restore the categories when you move the draft back into article space.

In cases where, for technical reasons, project categories appear directly on articles rather than talk pages, they should be made into hidden categories, so that they are not displayed to readers. This rule does not apply to stub categories or "uncategorized article" categories – these types are not hidden.

A logged-in user may elect to view all hidden categories, by checking "Show Hidden Categories" on the "Appearance" tab of My Preferences. Notice that "hidden" parent categories are never in fact hidden on category pages (although they are listed separately).

Many templates include category declarations in their transcludable text, for the purpose of placing the pages containing those templates into specific categories. This technique is very commonly used for populating certain kinds of project categories, including stub categories and maintenance categories.

However, it is recommended that articles should not be placed in ordinary content categories using templates in this way. There are many reasons for this – editors cannot see the category in the wikitext; removing or restructuring the category is made more difficult (partly because automated processes will not work); inappropriate articles and non-article pages may get added to the category; sort keys may be unavailable to be customised per category; ordering of categories on the page is less controllable; and the "incategory" search term will not find such pages [୧].

When templates are used to populate project categories, ensure that the code cannot generate nonsensical or non-existent categories, particularly when the category name depends on a parameter. Also, see Category suppression for ways of keeping inappropriate pages out of template-generated categories.

Category declarations in templates often use {{PAGENAME}} as the sort key, particularly if they are designed to be placed on talk pages, as this suppresses the Talk: prefix. Note that this overrides any DEFAULTSORT defined on the page.

Because of software limitations [୨], ordinary (hard) redirects should not be used with category pages. If a category name needs to be redirected to another, use the {{Category redirect}} template to create a soft redirect. A bot traverses categories redirected in this manner, moving articles out of the redirected category into the target category. (See Template talk:Category redirect.)